In a digital communication network, measurements of the accuracy of transmitted data are expressed in terms of several parameters. One of which is a bit error rate, i.e., the fraction of the received bits that are in error. This invention is directed to such an error rate measurement system to measure the bit error rate of an optical pulse signal train in an optical communication network.
Because of the needs of high speed and high density data transmission in the optical communication network, transmissions using optical soliton waves have been attracted attention in the communication industry. This is because a soliton signal in an optical fiber is an extremely good carrier of optical information because of its short duration and high stability which are the very characteristics suitable for optical time division multiplexing. The optical soliton system may realize soliton transmission for a distance of about 10,000 km without significant loss.
Therefore, there is a need of an error rate measurement apparatus for detecting errors in a high speed optical pulse train. However, the conventional error rate measurement instruments are relatively slow in the error detecting speed which is not sufficient for the soliton system whose frequency is typically 10-20 GHz. For example, a protocol analyzer in the ISDN (Integrated Service Digital Networks), which is one of the error rate measurement instruments, covers a measurement frequency up to 1 GHz (10.sup.9 bit/sec). For data pulses having a frequency higher than this level, an eye diagram method is used to detect errors therein.
In the eye diagram method, the data pulse is evaluated by observing an eye pattern displayed on a screen of a sampling oscilloscope and the like. In this arrangement, a data pulse train which has been experienced noises and distortions in a transmission path is sampled by a clock signal which is synchronized with the data pulse train having a lower frequency than that of the data pulse train. The sampled signal is displayed on the oscilloscope as an eye pattern.
The opening of the eye, i.e., the voltage difference between the upper voltage and the lower voltage, represents a signal-noise ratio. The greater the opening, the higher it means the signal-noise ratio. When the waveform of the data pulse is degraded, the opening of the eye pattern becomes small. When there is a phase jitter in the data pulse train, the width of the eye pattern becomes small. In this manner, even if the incoming pulse train has a high repetition rate, the errors are detected by the sampling pulse in the eye diagram method so long as the pulse train is continuous and repetitive.
As noted above, the optical soliton caused by a nonlinear action in an optical fiber could be used as an ideal method for a long distance and high density optical communication network. However, to realize such an optical communication network, various parameters must be evaluated including a bit error rate of the optical pulse. This is because there are several factors that cause deterioration in the quality of the optical pulse when transmitted through the optical fiber.
For example, when traveling the optical fiber, the optical pulse train may be phase modulated by the effect of the nonlinear refractive index of the optical fiber. Such a phase modulation interacts with the group velocity dispersion in the optical fiber, which causes the waveform deterioration in the pulse train. Optical amplifiers inserted during the path of the optical fiber may also adversely affect the transmission quality of the pulse train by their polarization dispersion.
The other cause of adversely affecting the waveform quality of the optical pulse train is a mixing of the optical signal light and optical noises in the optical fiber, i.e., a three wave mixing or a four wave mixing, which results in an abrupt increase of noise and a deterioration in the signal waveform. The three wave mixing or four wave mixing is a nonlinear process in which a third or a fourth output signal will be produced when two or three input signals are mixed in a nonlinear optical medium.
Further sources which adversely affect the quality of the optical pulse train are impurities of the materials used in an optical amplifier or an optical fiber, environmental changes such as pressure and temperature surrounding the optical fibers and other optical materials, which cause the polarization dispersion in the optical fiber.
In the conventional technology, it is not possible to measure the transmission quality of the optical pulse train, because the response characteristics of electrical circuits which receive electrical signals converted by a wide band optical detector are insufficient to fully respond to the repetition rate of the pulse train. An optical soliton pulse train is an ultra high repetition rate pulse signal, 10-20 GHz for example, and a pulse width of each optical pulse is less than several picosecond (10.sup.-12 second). Therefore, as noted above, the transmission quality is evaluated by the eye diagram method by sampling the input signal with sampling pulses of a lower repetition rate than the input signal and in synchronism with the input signal.
However, the eye diagram method is only effective for an input signal which is continuous with a constant repetition rate. It is not effective to evaluate the input signal transmitted through a long distance optical path in which pulses in the input signal may be lost or greatly deformed by the polarization dispersion due to the various causes as noted above. Thus, there is a need of a new type of bit error rate measurement system to directly measure a bit error rate of an optical soliton pulse train.